The worst trade I ever saw was on NBA 06. Dallas traded Dirk to Milwaukee for Michael Redd, which doesn't sound too bad at first.
Unfortunately, the computer never adjusted the rosters, using Redd as a PF that would continually get perpetually abused, to the point of Dallas not winning a single game for the remainder of the season.
My most insane trade was on NBA Shootout 98, trading Greg Ostertag for Shaq.